Categorising all the emails is not the challenge. The author needs to extract a very small number of genuine requests from the slush pile. which is a different problem, and much easier to set expectations in the community for.
I can completely understand not wanting to deal with the hassle, but pretending it's all about AI is disingenuous in my view.
The author needs to avoid a sufficient number of false positives for the time investment not to be prohibitive, and that is what he is arguing is becoming a hard problem. I have problems believing that given some of the e-mail I receive. I have no problem trusting Scalzi on this.